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1 | =head1 NAME |
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2 | X<warning, lexical> X<warnings> X<warning> |
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3 | |
4 | perllexwarn - Perl Lexical Warnings |
5 | |
6 | =head1 DESCRIPTION |
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7 | |
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8 | The C<use warnings> pragma is a replacement for both the command line |
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9 | flag B<-w> and the equivalent Perl variable, C<$^W>. |
10 | |
11 | The pragma works just like the existing "strict" pragma. |
12 | This means that the scope of the warning pragma is limited to the |
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13 | enclosing block. It also means that the pragma setting will not |
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14 | leak across files (via C<use>, C<require> or C<do>). This allows |
15 | authors to independently define the degree of warning checks that will |
16 | be applied to their module. |
17 | |
18 | By default, optional warnings are disabled, so any legacy code that |
19 | doesn't attempt to control the warnings will work unchanged. |
20 | |
21 | All warnings are enabled in a block by either of these: |
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22 | |
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23 | use warnings; |
24 | use warnings 'all'; |
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25 | |
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26 | Similarly all warnings are disabled in a block by either of these: |
27 | |
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28 | no warnings; |
29 | no warnings 'all'; |
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30 | |
31 | For example, consider the code below: |
32 | |
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33 | use warnings; |
34 | my @a; |
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35 | { |
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36 | no warnings; |
37 | my $b = @a[0]; |
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38 | } |
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39 | my $c = @a[0]; |
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40 | |
41 | The code in the enclosing block has warnings enabled, but the inner |
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42 | block has them disabled. In this case that means the assignment to the |
43 | scalar C<$c> will trip the C<"Scalar value @a[0] better written as $a[0]"> |
44 | warning, but the assignment to the scalar C<$b> will not. |
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45 | |
46 | =head2 Default Warnings and Optional Warnings |
47 | |
48 | Before the introduction of lexical warnings, Perl had two classes of |
49 | warnings: mandatory and optional. |
50 | |
51 | As its name suggests, if your code tripped a mandatory warning, you |
52 | would get a warning whether you wanted it or not. |
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53 | For example, the code below would always produce an C<"isn't numeric"> |
54 | warning about the "2:". |
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55 | |
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56 | my $a = "2:" + 3; |
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57 | |
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58 | With the introduction of lexical warnings, mandatory warnings now become |
59 | I<default> warnings. The difference is that although the previously |
60 | mandatory warnings are still enabled by default, they can then be |
61 | subsequently enabled or disabled with the lexical warning pragma. For |
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62 | example, in the code below, an C<"isn't numeric"> warning will only |
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63 | be reported for the C<$a> variable. |
64 | |
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65 | my $a = "2:" + 3; |
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66 | no warnings; |
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67 | my $b = "2:" + 3; |
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68 | |
69 | Note that neither the B<-w> flag or the C<$^W> can be used to |
70 | disable/enable default warnings. They are still mandatory in this case. |
71 | |
72 | =head2 What's wrong with B<-w> and C<$^W> |
73 | |
74 | Although very useful, the big problem with using B<-w> on the command |
75 | line to enable warnings is that it is all or nothing. Take the typical |
76 | scenario when you are writing a Perl program. Parts of the code you |
77 | will write yourself, but it's very likely that you will make use of |
78 | pre-written Perl modules. If you use the B<-w> flag in this case, you |
79 | end up enabling warnings in pieces of code that you haven't written. |
80 | |
81 | Similarly, using C<$^W> to either disable or enable blocks of code is |
82 | fundamentally flawed. For a start, say you want to disable warnings in |
83 | a block of code. You might expect this to be enough to do the trick: |
84 | |
85 | { |
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86 | local ($^W) = 0; |
87 | my $a =+ 2; |
88 | my $b; chop $b; |
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89 | } |
90 | |
91 | When this code is run with the B<-w> flag, a warning will be produced |
92 | for the C<$a> line -- C<"Reversed += operator">. |
93 | |
94 | The problem is that Perl has both compile-time and run-time warnings. To |
95 | disable compile-time warnings you need to rewrite the code like this: |
96 | |
97 | { |
98 | BEGIN { $^W = 0 } |
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99 | my $a =+ 2; |
100 | my $b; chop $b; |
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101 | } |
102 | |
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103 | The other big problem with C<$^W> is the way you can inadvertently |
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104 | change the warning setting in unexpected places in your code. For example, |
105 | when the code below is run (without the B<-w> flag), the second call |
106 | to C<doit> will trip a C<"Use of uninitialized value"> warning, whereas |
107 | the first will not. |
108 | |
109 | sub doit |
110 | { |
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111 | my $b; chop $b; |
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112 | } |
113 | |
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114 | doit(); |
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115 | |
116 | { |
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117 | local ($^W) = 1; |
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118 | doit() |
119 | } |
120 | |
121 | This is a side-effect of C<$^W> being dynamically scoped. |
122 | |
123 | Lexical warnings get around these limitations by allowing finer control |
124 | over where warnings can or can't be tripped. |
125 | |
126 | =head2 Controlling Warnings from the Command Line |
127 | |
128 | There are three Command Line flags that can be used to control when |
129 | warnings are (or aren't) produced: |
130 | |
131 | =over 5 |
132 | |
133 | =item B<-w> |
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134 | X<-w> |
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135 | |
136 | This is the existing flag. If the lexical warnings pragma is B<not> |
137 | used in any of you code, or any of the modules that you use, this flag |
138 | will enable warnings everywhere. See L<Backward Compatibility> for |
139 | details of how this flag interacts with lexical warnings. |
140 | |
141 | =item B<-W> |
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142 | X<-W> |
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143 | |
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144 | If the B<-W> flag is used on the command line, it will enable all warnings |
145 | throughout the program regardless of whether warnings were disabled |
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146 | locally using C<no warnings> or C<$^W =0>. This includes all files that get |
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147 | included via C<use>, C<require> or C<do>. |
148 | Think of it as the Perl equivalent of the "lint" command. |
149 | |
150 | =item B<-X> |
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151 | X<-X> |
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152 | |
153 | Does the exact opposite to the B<-W> flag, i.e. it disables all warnings. |
154 | |
155 | =back |
156 | |
157 | =head2 Backward Compatibility |
158 | |
159 | If you are used with working with a version of Perl prior to the |
160 | introduction of lexically scoped warnings, or have code that uses both |
161 | lexical warnings and C<$^W>, this section will describe how they interact. |
162 | |
163 | How Lexical Warnings interact with B<-w>/C<$^W>: |
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164 | |
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165 | =over 5 |
166 | |
167 | =item 1. |
168 | |
169 | If none of the three command line flags (B<-w>, B<-W> or B<-X>) that |
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170 | control warnings is used and neither C<$^W> or the C<warnings> pragma |
171 | are used, then default warnings will be enabled and optional warnings |
172 | disabled. |
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173 | This means that legacy code that doesn't attempt to control the warnings |
174 | will work unchanged. |
175 | |
176 | =item 2. |
177 | |
178 | The B<-w> flag just sets the global C<$^W> variable as in 5.005 -- this |
179 | means that any legacy code that currently relies on manipulating C<$^W> |
180 | to control warning behavior will still work as is. |
181 | |
182 | =item 3. |
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183 | |
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184 | Apart from now being a boolean, the C<$^W> variable operates in exactly |
185 | the same horrible uncontrolled global way, except that it cannot |
186 | disable/enable default warnings. |
187 | |
188 | =item 4. |
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189 | |
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190 | If a piece of code is under the control of the C<warnings> pragma, |
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191 | both the C<$^W> variable and the B<-w> flag will be ignored for the |
192 | scope of the lexical warning. |
193 | |
194 | =item 5. |
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195 | |
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196 | The only way to override a lexical warnings setting is with the B<-W> |
197 | or B<-X> command line flags. |
198 | |
199 | =back |
200 | |
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201 | The combined effect of 3 & 4 is that it will allow code which uses |
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202 | the C<warnings> pragma to control the warning behavior of $^W-type |
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203 | code (using a C<local $^W=0>) if it really wants to, but not vice-versa. |
204 | |
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205 | =head2 Category Hierarchy |
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206 | X<warning, categories> |
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207 | |
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208 | A hierarchy of "categories" have been defined to allow groups of warnings |
209 | to be enabled/disabled in isolation. |
210 | |
211 | The current hierarchy is: |
212 | |
213 | all -+ |
214 | | |
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215 | +- assertions |
216 | | |
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217 | +- closure |
218 | | |
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219 | +- deprecated |
220 | | |
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221 | +- exiting |
222 | | |
223 | +- glob |
224 | | |
225 | +- io -----------+ |
226 | | | |
227 | | +- closed |
228 | | | |
229 | | +- exec |
230 | | | |
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231 | | +- layer |
232 | | | |
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233 | | +- newline |
234 | | | |
235 | | +- pipe |
236 | | | |
237 | | +- unopened |
238 | | |
239 | +- misc |
240 | | |
241 | +- numeric |
242 | | |
243 | +- once |
244 | | |
245 | +- overflow |
246 | | |
247 | +- pack |
248 | | |
249 | +- portable |
250 | | |
251 | +- recursion |
252 | | |
253 | +- redefine |
254 | | |
255 | +- regexp |
256 | | |
257 | +- severe -------+ |
258 | | | |
259 | | +- debugging |
260 | | | |
261 | | +- inplace |
262 | | | |
263 | | +- internal |
264 | | | |
265 | | +- malloc |
266 | | |
267 | +- signal |
268 | | |
269 | +- substr |
270 | | |
271 | +- syntax -------+ |
272 | | | |
273 | | +- ambiguous |
274 | | | |
275 | | +- bareword |
276 | | | |
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277 | | +- digit |
278 | | | |
279 | | +- parenthesis |
280 | | | |
281 | | +- precedence |
282 | | | |
283 | | +- printf |
284 | | | |
285 | | +- prototype |
286 | | | |
287 | | +- qw |
288 | | | |
289 | | +- reserved |
290 | | | |
291 | | +- semicolon |
292 | | |
293 | +- taint |
294 | | |
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295 | +- threads |
296 | | |
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297 | +- uninitialized |
298 | | |
299 | +- unpack |
300 | | |
301 | +- untie |
302 | | |
303 | +- utf8 |
304 | | |
305 | +- void |
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306 | |
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307 | Just like the "strict" pragma any of these categories can be combined |
308 | |
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309 | use warnings qw(void redefine); |
310 | no warnings qw(io syntax untie); |
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311 | |
312 | Also like the "strict" pragma, if there is more than one instance of the |
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313 | C<warnings> pragma in a given scope the cumulative effect is additive. |
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314 | |
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315 | use warnings qw(void); # only "void" warnings enabled |
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316 | ... |
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317 | use warnings qw(io); # only "void" & "io" warnings enabled |
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318 | ... |
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319 | no warnings qw(void); # only "io" warnings enabled |
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320 | |
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321 | To determine which category a specific warning has been assigned to see |
322 | L<perldiag>. |
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323 | |
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324 | Note: In Perl 5.6.1, the lexical warnings category "deprecated" was a |
325 | sub-category of the "syntax" category. It is now a top-level category |
326 | in its own right. |
327 | |
328 | |
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329 | =head2 Fatal Warnings |
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330 | X<warning, fatal> |
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331 | |
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332 | The presence of the word "FATAL" in the category list will escalate any |
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333 | warnings detected from the categories specified in the lexical scope |
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334 | into fatal errors. In the code below, the use of C<time>, C<length> |
335 | and C<join> can all produce a C<"Useless use of xxx in void context"> |
336 | warning. |
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337 | |
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338 | use warnings; |
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339 | |
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340 | time; |
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341 | |
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342 | { |
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343 | use warnings FATAL => qw(void); |
344 | length "abc"; |
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345 | } |
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346 | |
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347 | join "", 1,2,3; |
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348 | |
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349 | print "done\n"; |
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350 | |
351 | When run it produces this output |
352 | |
353 | Useless use of time in void context at fatal line 3. |
354 | Useless use of length in void context at fatal line 7. |
355 | |
356 | The scope where C<length> is used has escalated the C<void> warnings |
357 | category into a fatal error, so the program terminates immediately it |
358 | encounters the warning. |
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359 | |
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360 | To explicitly turn off a "FATAL" warning you just disable the warning |
361 | it is associated with. So, for example, to disable the "void" warning |
362 | in the example above, either of these will do the trick: |
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363 | |
364 | no warnings qw(void); |
365 | no warnings FATAL => qw(void); |
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366 | |
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367 | If you want to downgrade a warning that has been escalated into a fatal |
368 | error back to a normal warning, you can use the "NONFATAL" keyword. For |
369 | example, the code below will promote all warnings into fatal errors, |
370 | except for those in the "syntax" category. |
371 | |
372 | use warnings FATAL => 'all', NONFATAL => 'syntax'; |
373 | |
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374 | =head2 Reporting Warnings from a Module |
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375 | X<warning, reporting> X<warning, registering> |
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376 | |
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377 | The C<warnings> pragma provides a number of functions that are useful for |
378 | module authors. These are used when you want to report a module-specific |
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379 | warning to a calling module has enabled warnings via the C<warnings> |
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380 | pragma. |
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381 | |
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382 | Consider the module C<MyMod::Abc> below. |
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383 | |
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384 | package MyMod::Abc; |
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385 | |
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386 | use warnings::register; |
387 | |
388 | sub open { |
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389 | my $path = shift; |
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390 | if ($path !~ m#^/#) { |
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391 | warnings::warn("changing relative path to /var/abc") |
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392 | if warnings::enabled(); |
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393 | $path = "/var/abc/$path"; |
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394 | } |
395 | } |
396 | |
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397 | 1; |
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398 | |
399 | The call to C<warnings::register> will create a new warnings category |
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400 | called "MyMod::abc", i.e. the new category name matches the current |
401 | package name. The C<open> function in the module will display a warning |
402 | message if it gets given a relative path as a parameter. This warnings |
403 | will only be displayed if the code that uses C<MyMod::Abc> has actually |
404 | enabled them with the C<warnings> pragma like below. |
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405 | |
406 | use MyMod::Abc; |
407 | use warnings 'MyMod::Abc'; |
408 | ... |
409 | abc::open("../fred.txt"); |
410 | |
411 | It is also possible to test whether the pre-defined warnings categories are |
412 | set in the calling module with the C<warnings::enabled> function. Consider |
413 | this snippet of code: |
414 | |
415 | package MyMod::Abc; |
416 | |
417 | sub open { |
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418 | warnings::warnif("deprecated", |
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419 | "open is deprecated, use new instead"); |
420 | new(@_); |
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421 | } |
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422 | |
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423 | sub new |
424 | ... |
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425 | 1; |
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426 | |
427 | The function C<open> has been deprecated, so code has been included to |
428 | display a warning message whenever the calling module has (at least) the |
429 | "deprecated" warnings category enabled. Something like this, say. |
430 | |
431 | use warnings 'deprecated'; |
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432 | use MyMod::Abc; |
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433 | ... |
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434 | MyMod::Abc::open($filename); |
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435 | |
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436 | Either the C<warnings::warn> or C<warnings::warnif> function should be |
437 | used to actually display the warnings message. This is because they can |
438 | make use of the feature that allows warnings to be escalated into fatal |
439 | errors. So in this case |
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440 | |
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441 | use MyMod::Abc; |
442 | use warnings FATAL => 'MyMod::Abc'; |
e476b1b5 |
443 | ... |
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444 | MyMod::Abc::open('../fred.txt'); |
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445 | |
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446 | the C<warnings::warnif> function will detect this and die after |
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447 | displaying the warning message. |
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448 | |
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449 | The three warnings functions, C<warnings::warn>, C<warnings::warnif> |
450 | and C<warnings::enabled> can optionally take an object reference in place |
451 | of a category name. In this case the functions will use the class name |
452 | of the object as the warnings category. |
453 | |
454 | Consider this example: |
455 | |
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456 | package Original; |
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457 | |
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458 | no warnings; |
459 | use warnings::register; |
7e6d00f8 |
460 | |
461 | sub new |
462 | { |
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463 | my $class = shift; |
464 | bless [], $class; |
7e6d00f8 |
465 | } |
466 | |
467 | sub check |
468 | { |
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469 | my $self = shift; |
470 | my $value = shift; |
7e6d00f8 |
471 | |
472 | if ($value % 2 && warnings::enabled($self)) |
473 | { warnings::warn($self, "Odd numbers are unsafe") } |
474 | } |
475 | |
476 | sub doit |
477 | { |
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478 | my $self = shift; |
479 | my $value = shift; |
480 | $self->check($value); |
7e6d00f8 |
481 | # ... |
482 | } |
483 | |
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484 | 1; |
7e6d00f8 |
485 | |
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486 | package Derived; |
7e6d00f8 |
487 | |
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488 | use warnings::register; |
489 | use Original; |
490 | our @ISA = qw( Original ); |
7e6d00f8 |
491 | sub new |
492 | { |
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493 | my $class = shift; |
494 | bless [], $class; |
7e6d00f8 |
495 | } |
496 | |
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497 | |
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498 | 1; |
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499 | |
500 | The code below makes use of both modules, but it only enables warnings from |
501 | C<Derived>. |
502 | |
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503 | use Original; |
504 | use Derived; |
7e6d00f8 |
505 | use warnings 'Derived'; |
63acfd00 |
506 | my $a = Original->new(); |
4358a253 |
507 | $a->doit(1); |
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508 | my $b = Derived->new(); |
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509 | $a->doit(1); |
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510 | |
511 | When this code is run only the C<Derived> object, C<$b>, will generate |
512 | a warning. |
513 | |
514 | Odd numbers are unsafe at main.pl line 7 |
515 | |
516 | Notice also that the warning is reported at the line where the object is first |
517 | used. |
518 | |
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519 | =head1 TODO |
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520 | |
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521 | perl5db.pl |
522 | The debugger saves and restores C<$^W> at runtime. I haven't checked |
523 | whether the debugger will still work with the lexical warnings |
524 | patch applied. |
525 | |
526 | diagnostics.pm |
527 | I *think* I've got diagnostics to work with the lexical warnings |
528 | patch, but there were design decisions made in diagnostics to work |
529 | around the limitations of C<$^W>. Now that those limitations are gone, |
530 | the module should be revisited. |
531 | |
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532 | document calling the warnings::* functions from XS |
533 | |
0453d815 |
534 | =head1 SEE ALSO |
535 | |
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536 | L<warnings>, L<perldiag>. |
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537 | |
0453d815 |
538 | =head1 AUTHOR |
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539 | |
0453d815 |
540 | Paul Marquess |